1. Field of the Invention
The present invention relates to apparatus for reducing pressure and temperature of steam in a steam line. More particularly, the present invention provides a steam conditioning valve for reducing the pressure and degree of superheat of steam in which the valve means includes a butterfly valve having water spray nozzles on its downstream face.
2. Background of the Prior Art
Sub-cooled spray water from a boiler feed pump is generally used in the power and process industry to cool superheated steam to its saturation temperature or to a desired degree of superheat. This is generally achieved by means of a mechanical or steam assisted desuperheater, equipped with either fixed or variable opening nozzles. Combining such a desuperheater with a steam pressure control valve results in a steam conditioning valve which simultaneously reduces pressure and temperature of superheated steam to a predetermined value.
There is a significant difference between a true steam conditioning valve and merely a pressure control valve in series with a desuperheater. In the "combined" steam conditioning valve, spray water injection is an integral part of the steam control valve and the valve is capable of mechanically proportioning the water mass flow as a function of the pressure reducing valve trim position. This direct water proportioning results in a valve which is particularly suitable for applications demanding fast reaction times, frequent load changes, cyclic operation, accurate temperature controls and no accidental over-spray.
A spray water control valve is generally used to supply correct inlet pressure to a steam conditioning valve and to fine-tune the water requirements based on downstream steam temperature measurement and a resulting instrument signal from the control loop. By contrast, steam pressure control valves with a desuperheating device downstream of the pressure reducing stage exhibit a much slower response time and have the potential to temporarily over-spray or under-spray. The degree of turbulence which is an important parameter from a mixing point of view is much lower than by injecting in the pressure reducing valve. The overall performance of these systems is frequently less than desirable and can cause operational headaches.
Basic desuperheating valve systems are typically based on a conventional in-line or angle body pressure reducing valve approach. These are normally plug valves in which the spray water is supplied through the valve stem and plug. Although these valves may function quite satisfactorily, simpler and more reliable steam conditioning valves are believed to be possible.
In addition to inaccurate delivery of desuperheating water, other problems which must be avoided in designing a practical steam conditioning valve include operational or maintenance problems due to thermal expansion, such as valve jamming and inadequate sealing.
Accordingly, it is a primary object of the present invention to provide a steam conditioning valve which efficiently combines temperature reduction and pressure reduction capabilities into a single unit.
It is a further object of the present invention to provide such a steam conditioning valve which is as simple and reliable as possible.
It is another object of the present invention to provide such a steam conditioning valve which provides accurate desuperheating while avoiding maintenance problems caused by thermal expansion/contraction, such as valve jamming and inadequate valve sealing.
It is another objective to achieve a maximum Cv for a minimum physical valve size and weight (typical for butterfly valves).
A further objective is to have a control element which is not sensitive to debris in the flow (example: welding rod ends, exfoliation products from superheater piping, welding dirt etc.)
These and other objects of the present invention will become evident from review of the following specification.